There exist in the patent literature various proposals for detection and warning of the presence of foreign objects on airport travel surfaces. The following patent documents are believed to represent the current state of the art.
U.S. Published Patent Applications Nos. 2002/0080046A1; 2002/0109625 A1 and 2002/0093433 A1.
Additionally, U.S. Pat. No. 6,064,429 deals with the detection of foreign objects in a general sense.